The head judge in his trial said Vu's writings and interviews "blackened" the Communist Party of Vietnam.

Former wartime foes Vietnam and the US have worked to improve ties in recent decades, but concerns over human rights have sometimes strained the relationship.

The one-party state is regularly denounced by rights groups and Western governments for its intolerance of political dissent and systematic violations of freedom of religion.

Human Rights Watch's Asia deputy director Phil Robertson said Vu's release was a "welcome development, especially given the serious health issues he developed while unjustly imprisoned by the Vietnam authorities."

"He should never have been tried or imprisoned in the first place because all he did was exercise his right to free speech," Robertson added.

Vietnam bans private media and all newspapers and television channels are state-run. Lawyers, bloggers and activists are regularly subject to arbitrary arrest and detention, according to rights groups.

Reporters Without Borders said earlier this month that Vietnam was second only to China in the number of bloggers it detained with at least 34 behind bars.